Free cytosolic calcium in renal proximal tubules from the spontaneously hypertensive rat

Free intracellular calcium was measured in renal proximal tubules obtained from spontaneously hypertensive rats (SHR) and from age-matched Wistar-Kyoto rats (WKY) ingesting a normal diet. Experiments were performed on renal proximal tubule suspensions using fura-2 to monitor cytosolic calcium. In 4-week-old rats, when systolic blood pressure was not significantly different between the two groups, renal proximal tubule cytosolic calcium was similar (143 ± 28 and 144 ± 15 nM, respectively). By the age of 5 weeks, cytosolic calcium increased significantly in both SHR and WKY (214 ± 24 and 262 ± 34 nM, respectively, p < 0.05). Calcium, however, was not significantly different between the two groups, even though at this age blood pressure was higher in SHR than in WKY. As compared with values in 4-week-old rats, cytosolic calcium was also found increased in tubules from both SHR and WKY aged 10 to 12 weeks (261 ± 42 and 279 ± 30 nM, respectively) and 20 to 24 weeks (263 ± 42 and 308 ± 28 nM, respectively). However, no significant differences in cytosolic calcium were found between SHR and WKY even though at these ages systolic blood pressure increased markedly in the SHR. Moreover, regression analysis failed to reveal a correlation between cytosolic calcium and blood pressure when data from either group of rats of all ages studied were pooled. Exposure to ouabain (10^-3 M) to inhibit Na⁺,K⁺-adenosine triphosphatase and increase intracellular sodium had no significant effect on cytosolic calcium in tubules from either SHR or WKY (260 ± 69 and 250 ± 45 nM, respectively). This finding suggests that a circulating ouabainlike factor is not likely to increase cytosolic calcium in renal proximal tubules from the SHR. Our data indicate that cytosolic calcium is not elevated in renal proximal tubules obtained from the SHR either before or during sustained hypertension.